The possibility of using a polymer for reversible optical storage, holographic or digital, was first suggested in 1984, using an azo dye (methyl orange) dispersed into poly(vinyl alcohol). Since then Eich and Wendorff have demonstrated reversible optical storage properties on some liquid crystalline polymer films (U.S. Pat. Nos. 4,896,292; 4,886,718 and 4,837,745). One was a homopolymer containing a p-cyanoazobenzene bonded as a side chain in a polycarbonate through an oxygen atom and a spacer of six methylene groups. The spacer allowed the mesogenic azobenzene moiety to move about and organize into a liquid crystalline phase. The other film was an acrylic copolymer containing two types of mesogenic units: a p-cyanoazobenzene bonded through an oxygen and a spacer of six methylene groups and a p-cyanophenylbenzoate bonded in a similar way. The azobenzene moiety was ca. 30% of the structural units. When exposed to laser beams, these films stored the information written on them. The writing was done in the nematic or in the glassy phase (the glass temperature, Tg, is about 30.degree. C.) and erasure could be achieved by heating the film above the clearing temperature. The mechanism postulated for the phenomenon was obviously related to the well-known trans-cis isomerization and was believed to involve a reorientation of the side groups brought about by the movement of the trans-cis isomerization. This explanation is very logical if a rotation mechanism is the main consideration in isomerization and a volume of ca. 0.25 nm.sup.3 is required to accommodate the change. A more detailed investigation on the above copolymer showed that the polarized laser light turns the optical axis of the liquid crystalline polymer perpendicular to the polarization plane. The same writing phenomenon was observed in an amorphous copolymer (methylacrylate with 25 mol % azo component), except that in this case the writing beam was inducing the alignment (Anderle et. al. Mahromol. Chem. Rapid. Comm. 1989, 10, 477). The azo side chain was designed to include a long enough spacer in order to allow for reorientation. Thus, it appears that laser light induces a trans-cis isomerization accompanied by reorientation and that the reorientation is maintained even after all the cis-isomers revert to the more stable trans form. The same anisotropy can be induced by laser writing onto gelatin films doped with azo dyes (Ebradidze and Mumaladze Appl. Optics 1990, 29, 466).
Recently, a Japanese group has published two studies on similar copolymers, where the azo component is either a dopant in a liquid crystalline copolymer or a side chain comonomer (Macromolecules 1990, 23, 42 and Makromol Chem 1991, 192, 215). All polymers were liquid crystalline, but the spacers introduced to allow for liquid crystallinity were sometimes shorter (as low as 2 methylene groups). This produces polymers with higher glass transition temperatures, and the laser writing is performed in the nematic phase or in the glassy state.
A further significant development in the field, appeared in Nature 1991, 351, 49 where it was reported that induction of orientation can be obtained with a polarized laser or just by rubbing the polymer film in a certain direction. This orientation can be transmitted to a liquid crystalline material adjacent to the oriented film. The film employed in this reference was a polyimide doped with a diazo dye. At about the same time, Stumpe et al (Makromol Chem Rapid Commun 1991, 12, 81) synthesized a methacrylate copolymer with 14 mol % azobenzene side chains and with a Tg of 60.degree. C., significantly higher than any of the previous polymers. They demonstrated that writing can be performed in the glassy state, and that the reorientation of the azobenzene moieties actually affects the neighbouring mesogenic groups, probably by the same mechanism as described in the Nature article. To erase the writing on that copolymer heating above the clearing temperature (84.degree. C.) was necessary. The conclusions that can be drawn from all these studies is that writing with a polarized laser onto a polymer film is being performed by a trans-cis isomerization accompanied by reorientation, and that the reorientation, perpendicular to the laser polarization is being maintained even after the cis-trans isomerization, which occurs spontaneously with a time constant of about 4 hours at room temperature (Wiesner et. al. Makromol Chem. 1990, 191, 2133). This reorientation is transmitted to the neighbouring mesogenic units which are not photosensitive. The writing is permanent (a 2 year stability was reported for a hologram at room temperature) unless heating above the glass transition temperature, or above the clearing temperature is performed. The liquid crystalline nature of the polymers involved helps in allowing enough flexibility for the azo side group to reorient after the isomerization. On the other hand, spacers long enough to allow movement decrease the glass transition temperature of the film, and probably increases the rate of relaxation at room temperature. In principle, the higher the Tg of the polymer, the greater the stability of the writing at room temperature (well below Tg).
There is great interest in optical storage media which in addition to high recording densities are also capable of reversible storage. However, the polymer liquid crystals for use in such storage media described above are relatively costly to produce and have certain critical operating parameters. There is, therefore a need for improved polymers which do not rely on the liquid crystal phenomenon for use in optical data storage media, which can be written on and erased in a much simpler way, and which are simpler and cheaper to produce.